Q4 of 46 Page 249

State the law of conservation of momentum. Two billion people jump above the earth’s surface with a speed of 4m/s from the same spot. The mass of the earth is 6x1024 kg.

The average mass of one person is 60 kg.


i) What is the total momentum of all the people?


ii) What will be the effect of this action on the earth?

The law of conservation of momentum states that there is no change in momentum in an isolated system.

(i)Total momentum = mass × velocity = 108×60×4 = 2.4×1010kgm/s


(ii)According to the law of conservation of momentum, the earth will gain the same momentum in the opposite direction so that there is no change in the momentum of the system (total momentum remains zero).


So the recoil velocity of the Earth =


= 4 × 10-15 m/s


This velocity is negligibly small. So there’s no observable effect.


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